Firing head with detonation gas release activation

ABSTRACT

A firing head sub for use in explosive well perforation is described. The firing head initiates the firing of a charge within a downhole perforating gun, and also releases the firing head and perforating gun from the tubing string upon firing so as to avoid clogging of the perforating gun with sand and jamming of the perforating gun downhole. The firing head and perforating gun will only be released upon detonation of the charge. This is ensured by a feedback loop that returns spent detonation gas to the firing head to shift a sleeve, thereby unbiasing the firing head latches from their engagement with the tubing string connector, thereby releasing the firing head from the tubing string.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Canadian Application No. 2,470,562, filed Jun. 9, 2004, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to well perforation tools. More particularly, the present invention relates to firing head subs for use in explosive well perforation.

BACKGROUND OF THE INVENTION

During the production life of an oil or gas well, it often becomes necessary to perforate the well casing and cement liner of the well in order to obtain access to further deposits within the formation. Several technologies have been developed for this purpose, including the use of downhole explosives, which can be generally targeted to perforate the casing at a specific well depth. Generally, a “gun” containing an explosive charge, and a firing head are attached to the bottom of a tubing string and are lowered to the target perforation depth. A mechanical stimulus, such as air or water pressure, is applied to the firing head in order to detonate the charge.

In a typical low viscosity formation, the oil or gas will flow easily from the formation through the perforations in the well, into the tubing string and past the gun, where it then bypasses the tubing string and firing head to flow to the surface. However, when the formation comprises sands, production is impaired by the presence of the gun within the well's bore. Moreover, explosive perforation in oil sands often results in the gun clogging with sand or becoming lodged within the tubing string. In such situations, removal of the entire tubing string is necessary to dislodge and clear the gun from the bore.

Automatic release of the firing head and gun from the tubing string connector is effected in prior art and other art in the field in response to the activation which simultaneously causes gun firing. Therefore, the release of the tool in those devices will occur regardless of whether the entire perforating charge in the gun is detonated. In other words, even if the charge fails to fully detonate, the gun and firing head will be released from the tubing string connector. In certain situations, it may be desirable to retain the gun and firing head attached to the tubing string connector if the charge fails to fully detonate, thereby providing an indication to the user that further perforation may be required, or that the failure to flow was related to the perforation event not occurring.

It is, therefore, desirable to provide a downhole tool which will detonate a well-perforating charge, wherein only a substantially full charge detonation results in release of the perforating gun and firing head from the tubing string connector.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate at least one disadvantage of previous firing head subs.

A firing head sub for use in explosive well perforation is described. The firing head initiates the firing of a charge within a downhole perforating gun, and also releases the firing head and perforating gun from the tubing string upon firing so as to avoid clogging of the perforating gun with sand and jamming of the perforating gun downhole. The firing head and perforating gun will only be released upon detonation of the charge. This is ensured by a feedback loop that returns spent detonation gas to the firing head to shift a sleeve, thereby unbiasing the firing head latches from their engagement with the tubing string connector, thereby releasing the firing head from the tubing string.

In a first aspect, the present invention provides an automatic release firing head sub for attachment between a tubing string and perforating gun to be fired by a charge initiator having a tubing string connector adapted to connect a firing head to the tubing string, a firing piston having a firing pin, the firing piston slideably engaged within a cylinder and drivable towards the charge initiator, a piston retainer for applying a retaining force to the firing piston to maintain the firing piston in a non-firing position, a firing mechanism activated by providing pressurized fluid to a side of the cylinder to drive the piston to overcome the retaining force of the piston retainer, thereby releasing and energizing the firing piston and firing pin to strike the charge initiator, a feedback channel for delivering detonation gas under high pressure from the perforating gun to the firing head, and a latching system for releasably attaching the firing head to the tubing string, the latching system unlatching only in response to the delivery of pressured detonation gas to the firing head. Preferably, the latching system includes a collar of flexible fingers, the fingers held by a retention sleeve to engage a recess within the tubing string connector while latched, but biased to disengage when the retention sleeve is moved. Preferably, the retention sleeve is slideably disposed between the cylinder and the fingers, and is held in place by a shearable link. Preferably, the retention sleeve is forced from between the cylinder and the fingers by the detonation gas exhausted through the feedback channel. Preferably, the shearable link comprises a shear pin extending between the tubing string connector and the retention sleeve. Preferably, the cylinder is a drive housing.

In a further aspect, the present invention provides a firing head sub for releasably retaining a perforating gun, the firing head sub having a feedback channel adapted to receive detonation gas under pressure from the perforating gun, and a latching system adapted to releasably retain the perforating gun, the latching system adapted to unlatch in response to the pressure of the detonation gas, wherein the latching system is adapted to unlatch when the detonation gas pressure reaches a threshold pressure.

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

FIG. 1 is a longitudinal cross sectional view of a portion of a downhole tool in accordance with an embodiment of the invention; and

FIG. 2 is an exploded view of the embodiment described in FIG. 1.

DETAILED DESCRIPTION

It is an object of the invention to address at least one of the shortcomings of prior downhole detonation systems.

Generally, the present invention provides a device for causing detonation of an explosive charge. More specifically, the invention provides a downhole firing head which activates the detonation of a series of at least one shaped explosive charge within a downhole perforating gun to perforate a well's casing to production zone, and also causes the gun to be released or ejected from the tubing string, but only following reasonably successful charge detonation.

Referring to FIGS. 1 and 2, an embodiment of the invention is disclosed in which the firing head and perforating gun are both released and propelled from the tubing string by the gas pressure produced upon detonation of the perforating charges. In this embodiment, the detonation gas from substantially all perforating charges is required to release the firing head and perforating gun from the tubing string. Such a system would enable an operator at the surface to recognize whether the well has been properly perforated by determining whether or not the head and perforating gun had been released from the tubing string.

The firing head sub 5 includes a firing head 10 attached to a tubing string connector 20 and to a perforating gun (not shown) containing explosive charges (not shown). The firing head 10 includes a firing piston 101, housed within a drive housing 103, which transmits a mechanical activating force to a detonation initiator 102, thereby supplying the impact or force required to initiate detonation of a charge and perforation of the well by the gun's shaped charges below the device. Following detonation, the spent detonation gas is captured within the firing head and effects release of the firing head and perforating gun (not shown) from the tubing string connector 20.

More specifically, when the tool is assembled, the drive housing 103 is inserted between the notched fingers 120 of the firing head and the drive housing 103 is threadably attached to the base of the firing head 10 near the detonation initiator 102. A retention sleeve 104 is slideably engaged between the drive housing 103 and the fingers 120 of the firing head 10 and is held in place by a shear pin 105 which is retained in place by a shear pin retainer 106. The shear pin 105 ensures that the retention sleeve 104 maintains its longitudinal position between the drive housing 103 and the fingers 120 in order to bias the notched fingers 120 to engagement with a corresponding recess 201 within the tubing string connector 20. A set screw 202 is set within a U-shaped groove 203 to restrain the firing head 10 from turning or rotating in relation to the tubing string connector 20, and to prevent accidental detonation of the charge.

When a mechanical activating force is applied to chamber A, the pressure inside chamber A will increase until it exceeds the pressure below the firing piston 101 and provides enough force to the firing piston 101 to exceed the designed tensile breaking point at a weak point 111 of a piston retainer 110. This breakage will permit the firing piston 101 to move towards the detonation initiator 102 in response to the high pressure within chamber A. Rapid movement of the firing piston 101 will, in turn, apply pressure to the detonation initiator 102, initiating firing of the perforating gun (not shown) and detonation of the explosive charges (not shown).

The tool of FIGS. 1 and 2 includes a detonation gas feedback loop including channels 301 and 302, by which high pressure spent detonation gas is returned to the firing head 10. Following detonation of the charges, the spent detonation gas is captured within the firing head 10 and is transmitted to one side of the retention sleeve 104 through gas flow channels 301, 302. As a result, the pressure within channels 301, 302 will build, and when substantially all charges have been detonated, or at least enough to reach a pre-selected threshold have been detonated, the pressure of the detonation gas within channels 301, 302 will overcome the shear pin 105 permitting movement of the retention sleeve 104 from between the drive housing 103 and the fingers 120. The fingers 120, no longer constrained to the recess 201, will then collapse, and the entire firing head 10 will therefore be released from the tubing string connector 20. In addition, the detonation gas pressure and the pressure in the tubing string combine to energize or propel the movement of the firing head 10 and all connected devices including the perforating gun, further into the well, clearing the newly perforated region for free flow of production or injection. Sand and other debris will not clog or jam the perforating gun in place at the perforations, and thus production or injection may begin immediately upon perforation.

In the tubing string connector 20, the feedback loop and/or the weak point 111 of the piston retainer 110, can be designed to release the firing head 10 when any given number of charges have been detonated, but it is preferable that the feedback loop activate the release of the firing head and gun only when substantially all charges have been detonated to indicate to the operator that, if no fluid flows, the well is either perforated or not perforated (and thus dry or not).

Referring to FIG. 2, a number of components are used to facilitate overall sealing and assembly of the firing head sub 5, including seals 400, o-rings 40, clip 420, and holder 430.

As the present system does not rely on the weight of the perforating gun or firing head to cause release of the perforating gun and firing head 10, but relies on controlling the bias against the notched fingers 120 of the firing head 10, the tool may actually permit the pressure of the delivered detonation gas through the tool to propel the gun away from the tubing string. This ensures that the gun will be clear of the perforated zone to permit production of oil and gas from the perforated well.

The above-described embodiments of the present invention are intended to be examples only. Alterations, modifications and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely by the claims appended hereto. 

1. A automatic release firing head sub for attachment between a tubing string and perforating gun to be fired by a charge initiator comprising: a. a tubing string connector adapted to connect a firing head to the tubing string; b. a firing piston having a firing pin, the firing piston slideably engaged within a cylinder and drivable towards the charge initiator; c. a piston retainer for applying a retaining force to the firing piston to maintain the firing piston in a non-firing position; d. a firing mechanism activated by providing pressurized fluid to a side of the cylinder to drive the piston to overcome the retaining force of the piston retainer, thereby releasing and energizing the firing piston and firing pin to strike the charge initiator; e. a feedback channel for delivering detonation gas under high pressure from the perforating gun to the firing head; and f. a latching system for releasably attaching the firing head to the tubing string, the latching system unlatching only in response to the delivery of detonation gas to the firing head.
 2. The firing head sub of claim 1 wherein the latching system includes a collar of flexible fingers, the fingers held by a retention sleeve to engage a recess within the tubing string connector while latched, but biased to disengage when the retention sleeve is moved.
 3. The firing head sub of claim 2, wherein the retention sleeve is slideably disposed between the cylinder and the fingers, and is held in place by a shearable link.
 4. The firing head sub of claim 3, wherein the retention sleeve is forced from between the cylinder and the fingers by the detonation gas exhausted through the feedback channel.
 5. The firing head sub of claim 3, wherein the shearable link comprises a shear pin extending between the tubing string connector and the retention sleeve.
 6. The firing head sub of claim 1, wherein the cylinder is a drive housing.
 7. A firing head sub for releasably retaining a perforating gun, the firing head sub comprising: a. a feedback channel adapted to receive detonation gas from the perforating gun; and b. a latching system adapted to releasably retain the perforating gun, the latching system adapted to unlatch in response to the detonation gas, wherein the latching system is adapted to unlatch when the detonation gas pressure reaches a threshold pressure.
 8. A method of moving a perforating gun on a tubing string from a perforation zone of a wellbore comprising the steps of: a. activating the perforating gun to generate detonation gases; and b. directing the detonation gases to trigger the disconnection of the perforating gun and the tubing string, wherein, the detonation gases must reach a threshold pressure in order to trigger the disconnection of the perforating gun and the tubing string.
 9. The method of claim 8, further comprising the step of propelling the perforating gun away from the tubing string, at least in part, using the detonation gases. 